The present invention relates to mounting and connection devices and techniques for use with microelectronic elements such as semiconductor chips.
Complex microelectronic devices such as modern semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require many hundreds of connections to external devices.
Semiconductor chips commonly have been connected to electrical traces on mounting substrates by one of three methods: wire bonding, tape automated bonding, and flip-chip bonding. In wire bonding, the chip is positioned on a substrate with a bottom or back surface of the chip abutting the substrate and with the contact-bearing front or top surface of the chip facing upwardly, away from the substrate. Individual gold or aluminum wires are connected between the contacts on the chip and pads on the substrate. In tape automated bonding a flexible dielectric tape with a prefabricated array of leads thereon is positioned over the chip and substrate and the individual leads are bonded to the contacts on the chip and to pads on the substrate. In both wire bonding and conventional tape automated bonding, the pads on the substrate are arranged outside of the area covered by the chip, so that the wires or leads fan out from the chip to the surrounding pads. The area covered by the subassembly as a whole is considerably larger than the area covered by the chip. This makes the entire assembly substantially larger than it otherwise would be. Because the speed with which a microelectronic assembly can operate is inversely related to its size, this presents a serious drawback. Moreover, the wire bonding and tape automated bonding approaches are generally most workable with chips having contacts disposed in rows extending along the periphery of the chip. They generally do not lend themselves to use with chips having contacts disposed in a so-called area array, i.e., a grid-like pattern covering all or a substantial portion of the chip front surface.
In the flip-chip mounting technique, the contact bearing surface of the chip faces towards the substrate. Each contact on the chip is joined by a solder bond to the corresponding pad on the substrate, as by positioning solder balls on the substrate or chip, juxtaposing the chip with the substrate in the front-face-down orientation and momentarily melting or reflowing the solder. The flip-chip technique yields a compact assembly, which occupies an area of the substrate no larger than the area of the chip itself. However, flip-chip assemblies suffer from significant problems with thermal stress. The solder bonds between the chip contacts and substrate are substantially rigid. Changes in the size of the chip and of the substrate due to thermal expansion and contraction in service create substantial stresses in these rigid bonds, which in turn can lead to fatigue failure of the bonds. Moreover, it is difficult to test the chip before attaching it to the substrate, and hence difficult to maintain the required outgoing quality level in the finished assembly, particularly where the assembly includes numerous chips.
Numerous attempts have been made to solve the foregoing problem. Useful solutions are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266. Preferred embodiments of the structures disclosed in these patents incorporate flexible, sheet-like structures referred to as xe2x80x9cinterposersxe2x80x9d or xe2x80x9cchip carriersxe2x80x9d. The preferred chip carriers have a plurality of terminals disposed on a flexible, sheet-like top layer. In use, the interposer is disposed on the front or contact bearing surface of the chip with the terminals facing upwardly, away from the chip. The terminals are then connected to the contacts of the chip. Most preferably, this connection is made by bonding prefabricated leads on the interposer to the chip contacts, using a tool engaged with the lead. The completed assembly is then connected to a substrate, as by bonding the terminals of the chip carrier to the substrate. Because the leads and the dielectric layer of the chip carrier are flexible, the terminals on the chip carrier can move relative to the contacts on the chip without imposing significant stresses on the bonds between the leads and the chip, or on the bonds between the terminals and the substrate. Thus, the assembly can compensate for thermal effects. Moreover, the assembly most preferably includes a compliant layer disposed between the terminals on the chip carrier and the face of the chip itself as, for example, an elastomeric layer incorporated in the chip carrier and disposed between the dielectric layer of the chip carrier and the chip. Such a compliant structure permits displacement of the individual terminals independently towards the chip. This permits effective engagement between the subassembly and a test fixture. Thus, a test fixture incorporating numerous electrical contacts can be engaged with all of the terminals in the subassembly despite minor variations in the height of the terminals. The subassembly can be tested before it is bonded to a substrate so as to provide a tested, known, good part to the substrate assembly operation. This in turn provides very substantial economic and quality advantages.
Co-pending, commonly assigned U.S. patent application Ser. No. 08/190,779 describes a further improvement. Components according to preferred embodiments of the ""779 application use a flexible, dielectric top sheet having top and bottom surfaces. A plurality of terminals are mounted on the top sheet. A support layer is disposed underneath the top sheet, the support layer having a bottom surface remote from the top sheet. A plurality of electrically conductive, elongated leads are connected to the terminals on the top sheet and extend generally side by side downwardly from the terminals through the support layer. Each lead has a lower end at the bottom surface of the support layer. The lower ends of the leads have conductive bonding materials as, for example, eutectic bonding metals. The support layer surrounds and supports the leads.
Components of this type can be connected to microelectronic elements such as semiconductor chips or wafers by juxtaposing the bottom surface of the support layer with the contact-bearing surface of the chip so as to bring the lower ends of the leads into engagement with the contacts on the chip, and then subjecting the assembly to elevated temperature and pressure conditions. All of the lower ends of the leads bond to the contacts on the chip substantially simultaneously. The bonded leads connect the terminals of the top sheet with the contacts on the chip. The support layer desirably is either formed from a relatively low-modulus, compliant material, or else is removed and replaced after the lead bonding step with such a compliant material. In the finished assembly, the terminals desirably are movable with respect to the chip to permit testing and to compensate for thermal effects. However, the components and methods of the ""779 application provide further advantages, including the ability to make all of the bonds to the chip or other component in a single lamination-like process step. The components and methods of the ""779 application are especially advantageous when used with chips or other microelectronic elements having contacts disposed in an area array.
Despite these and other advances in the art, there are still needs for further improvements.
One aspect of the present invention provides methods of making microelectronic lead arrays. Method according to this aspect of the present invention includes the steps of providing a first element having a first surface with a plurality of elongated, flexible leads extending along the first surface, each such lead having a terminal end attached to the first element and a tip end offset from the terminal end in a preselected, first horizontal direction parallel to the first surface. The method also includes the step of simultaneously forming all of the leads by moving all of the tip ends of the leads relative to the terminal ends thereof and relative to the first element so as to bend the tip ends away from the first element.
Desirably, the tip ends of all the leads are attached to a second element, and the step of moving the tip ends of the lead relative to the terminal ends of the leads includes the step of moving the second element relative to the first element. Thus, the second element may be moved, relative to the first element, in a second horizontal direction opposite to the first horizontal direction, i.e., opposite to the terminal end to tip end direction of the leads. The second element preferably also moves in a downward vertical direction, away from the first element, simultaneously with the horizontal movement. The net effect is to move the tip end of each lead horizontally towards its own terminal end and vertically away from the terminal end, thereby deforming the leads towards formed positions in which the leads extend generally vertically downwardly, away from the first element. Methods according to this aspect of the present invention preferably also include the step of injecting a flowable, desirably compliant dielectric material around the leads after the lead-forming step and then curing the flowable material so as to form a dielectric support layer around the leads.
Most preferably, the first element is a flexible, dielectric top sheet having terminal structures extending therethrough at the terminal ends of the leads. The forming process may be used to produce a component for subsequent attachment to a microelectronic element. In processes for making such a component, the second element may be a temporary, removable element as, for example, a sheet of a soluble polymer. This temporary element is removed after the support layer is formed, as by dissolving away the soluble sheet, leaving the tip ends of the leads exposed at the bottom surface of the support layer. Bonding material can be applied to the tip ends of the leads before or after the step of forming the dielectric support layer. The resulting component can be assembled to a chip or other microelectronic element by juxtaposing the exposed surface of the support layer with the contact bearing surface of the element and bonding the tip ends of the leads to the contacts of the chip or other element.
Thus, after the component is connected to the chip or other element, the terminals on the flexible sheet are electrically connected to the contacts of the chip, but are movable with respect to the contacts both in directions parallel to the surface of the chip and towards the surface of the chip. The resulting assembly can be tested readily by engagement with a test probe and can also be assembled readily to a larger substrate. The movability of the terminals will provide compensation for differences in thermal expansion and contraction of the chip and the substrate to which it is mounted.
In a variant of this approach, the second element is a permanent, flexible dielectric sheet, initially positioned adjacent the first sheet. The tip end of each lead is provided with a conductive tip structure, such as a conductive post or via extending through the second dielectric sheet. In the lead-forming step, the second sheet moves away from the first sheet, and the flowable dielectric material is injected between the sheets. The tip structures may be provided with conductive bonding materials, and the resulting component may be connected to a microelectronic element by juxtaposing the surface of the second sheet with the contact-bearing surface of the microelectronic element.
According to a further, and particularly preferred arrangement, the second element is itself a microelectronic element such as a semiconductor chip or wafer. In this arrangement, the step of attaching the tip ends of the leads to the second element includes the step of bonding the tip ends of the leads to the contacts on the chip or other microelectronic element. This step desirably is performed while the leads are in their initial, undeformed positions. Thus, all of the tip ends are bonded simultaneously to all of the contacts on the microelectronic element. Because the leads are in their initial, undeformed positions when bonded to the contacts, the tip positions of the lead tips are well controlled at this stage. This facilitates registration of the lead tips with the contacts. Further, the process lends itself to application of substantial forces between the lead tips and the contacts.
In a particularly preferred arrangement, the second element is a multi-chip unit such as a wafer incorporating a plurality of semiconductor chips having contacts thereon and the first element or sheet extends over a plurality of these chips so that the sheet includes a plurality of regions, one such region corresponding to each such chip. In this arrangement, the step of attaching the tip ends of the leads to the second element preferably includes the step of bonding the tip ends of leads in a plurality of such regions, and desirably in all of such regions, to the contacts on the corresponding chips simultaneously so that each such region is connected to one chip. The method further includes the step of severing the chips from the multichip element or wafer and severing the regions from the sheet so as to form individual units, each including one chip and the associated region of the sheet. Preferably, the method also includes the step of injecting a flowable dielectric material between the wafer and the sheet and curing the dielectric material to form a compliant dielectric support layer after the lead bonding step but before the severing step. The severing step thus includes the step of severing the dielectric support layer so that each unit formed in the severing step will include a portion of the dielectric support layer. Alternatively, the multi-chip unit may include an assembly of chips in the desired configuration for use, such as an assembly of chips mounted to a common heat sink or support, and the first element may include circuitry adapted to interconnect the plural chips. In this variant, the chips are not severed from one another.
Most preferably, the step of bonding the tip ends of the leads to the contacts of the microelectronic element includes the steps of aligning the top sheet or first element with the microelectronic element so that the tips are in registration with the contacts and biasing the sheet towards the microelectronic element while maintaining the registration. Thus, the sheet may be in engagement with a reinforcing structure during the bonding step to aid registration. The reinforcing structure may include a flexible but substantially inextensible foil such as a metallic foil bonded to the sheet. Alternatively or additionally, the reinforcing structure may include a substantially rigid ring having a central opening such that the sheet extends across the central opening and is held taut by the ring. The step of biasing the sheet towards the contact bearing surface may include the step of applying fluid pressure, such as air pressure, to the top surface of the sheet either directly or through a diaphragm or bag so as to maintain uniform pressure over the entire surface of the sheet.
A further aspect of the present invention provides a component for making a microelectronic connection including a first dielectric element having a first or bottom surface and a plurality of elongated flexible leads overlying the first surface of the first element. Each such lead has a terminal end secured to the first element and a tip end movable away from the first element, the tip end of each such lead being offset from the terminal end of the lead in a first horizontal direction parallel to the first surface. Preferably, the dielectric first element is a sheet having a top or second surface opposite from the first surface, and the component also includes electrically conductive terminal structures extending through the sheet at the terminal ends of the leads. Preferably, each lead has an electrically conductive bonding material at the tip end. Thus, the leads may include gold and the bonding material may include a metal selected to form a low-melting eutectic with gold, such as a metal selected from a group consisting of tin, germanium and silicon. The leads desirably are arranged in a regular, grid-like pattern at spacing intervals of less than about 1.25 mm between corresponding features of adjacent leads. Desirably, each lead is between about 200 and about 1000 microns long, about 10 microns to about 25 microns thick and about 10 microns to about 50 microns wide. Components according to this aspect of the invention can be used in methods as discussed above.
Yet another aspect of the invention provides a microelectronic connector including a body such as a flexible dielectric sheet having a bottom surface, there being an area array of terminal structures mounted to the body and exposed on the bottom surface thereof. The connector according to this aspect of the invention includes a plurality of leads, each said lead extending away from said bottom surface, each said lead having a terminal end connected to one said terminal structure and a tip end remote from the terminal structure. The connector according to this aspect of the invention further includes a layer of a compliant dielectric material on said bottom surface of said body, said compliant layer having a bottom surface remote from said body. The compliant layer substantially surrounds and support the leads. The tip ends of the leads protrude from the bottom surface of the compliant layer. Thus, the tip ends of the leads can be engaged with contacts on a microelectronic element by juxtaposing the contact-bearing surface of the microelectronic element with the bottom surface of the compliant layer. Each lead desirably has an electrically conductive bonding material at its tip end for joining the tip end to a contact.
The leads may be generally S-shaped. Each lead may be formed from a ribbon of conductive material having oppositely-directed major surfaces, the ribbon being curved in directions normal to its major surfaces to form the S-shape or other curved configuration of the lead.
Yet another aspect of the invention provides a microelectronic assembly incorporating a microelectronic element having a front surface with an area array of contacts thereon. The assembly includes a connector body having a bottom surface facing toward said front surface of said element but spaced therefrom. The connector body has an area array of terminal structures exposed on said bottom surface and overlying the array of contacts on the microelectronic element.
The assembly includes curved, preferably S-shaped flexible leads extending between the terminal structures and contacts. Here again, each flexible lead desirably is constituted by a metallic ribbon having oppositely-directed major surfaces, the ribbon being curved in directions normal to its major surfaces to form the S-shape.
Assemblies and connectors according to the last-said aspects of the invention can be fabricated readily by the preferred processes discussed above. Preferred assemblies and connectors according the these aspects of the invention provide compact, reliable connections for semiconductor chips and similar elements.